Hi Felix,
[snip]
This happens because the menu and submenu are part of
the Panel document (SpaceLinks), so the extended submenu can't appear out of this
panel document.
No. I think you're mixing the notion of an HTML document with the notion
of an XWiki document. Even though the panels are separate XWiki
documents, when a page is rendered the HTML representation of the panels
and the HTML representation of the page content are included in the same
HTML document.
I think you just have to play with the CSS (position, z-index) as
Jeremie said.
Hope this helps,
Marius